Marco Tech Blog

January 8, 2008

Handbook Note 74/113: PChar to String

Note 74 of 113 of Delphi 2007 Handbook

posted by marcocantu @ 10:48AM | 5 Comments [0 Pending]

Handbook Note 74/113 PChar to String 

I don't understand this note without seeing the code
it's referring to.  Would you be able to post that
line of code (or a similar one) please?

I think the source of my confusion is that I thought
the string type could handle appending a C-string and
the final resulting (native Delphi) string would have
the correct length.  So... how or why would this not
be the case?  Is this passing a string into an API
call to be /written to/, not read from, maybe?

Comment by David M [] on January 9, 03:21

Handbook Note 74/113 PChar to String 

David,

  good point... and yes, this is a case in which you
pass a PChar to a Windows API for writing, not
reading. This is the original code snippet the note
comments on:

var
  szBuffer: string;
begin
  SetLength (szBuffer, 250);
  SHGetFolderPath (Handle, CSIDL_PERSONAL,
    TOKEN_DEFAULTUSER, 0, PChar(szBuffer));
  szBuffer := PChar(szBuffer);

Comment by Marco Cantù [http://www.marcocantu.com] on January 9, 10:36

Handbook Note 74/113 PChar to String 

I'd use

SetLength (Buffer, MAX_PATH);

To ensure the buffer is sized up to the max path size 
used by the version of Windows the application is 
running on - and try to avoid buffer overflows.
 

Comment by Luigi D. Sandon on January 9, 13:43

Handbook Note 74/113 PChar to String 

Thanks Marco!  I didn't actually realise you could do
that - I've always passed in 'genuine' PChars, ie as
pointers to my own character buffer and created a
string from that after the function writing to it is
called.  Guess you learn something every day :)

Comment by David M on January 10, 01:05

Handbook Note 74/113 PChar to String 

 Even better is this :

procedure Example_WINAPI_returns_length;
var
  s: string;
begin
  // First, call the API without a buffer, to get the 
length) :
  SetLength(s, WINAPI({buffer=}nil, {length=}0));
  // Now, test if the call is needed anyway :
  if Length(s) > 0 then
    // Call it with the supplied buffer, and update 
the string-length for good measure :
    SetLength(s, WINAPI({buffer=}@(s[1]), {length=}
Length(s)));
end;

Comment by Patrick van Logchem [http://www.every-angle.com] on January 10, 22:35